Set Default MPEG Preseek
There is an addon called Property Chart



This addon adds a panel to the properties sidebar that allows you to easily copy settings between items. This addon works in the 3dview as well as the VSE.



By adding mpeg_preseek to the list, the value is then shown for the selected items and by setting the top item to the desired value you can then click the clipboard and have the value copied to all other items. One catch is that you need to set the value in the top item, if the top item doesn't have mpeg_preseek then you can't set it. This only matters as the last item selected when you select all will be on top of the list, having a transform selected first will then stop this working, so be sure to select a movie strip then select all.

A small script could also be used to set them. With a little effort this can be adjusted to add a properties panel with a button to set all strips to the desired value.

import bpy

for seq in bpy.context.scene.sequence_editor.sequences:
    if hasattr(seq, 'mpeg_preseek'):
        seq.mpeg_preseek = 25